When covering long distances men dehydrate faster than women. Dutch physiologists at the Radboud University Nijmegen discovered that men drink less and probably lose more fluids through transpiration than women. Men are four times as likely to suffer from dehydration than women.

The physiologists studied a total of 99 male and female participants aged between 21 and 82. They all took part in the annual International Four Days Marches, the largest multi-day walking event in the world. Three-quarters of the subjects exercised every day. The participants walked 30, 40 or 50 km.

As the first figure below on the left shows, the male participants lost twice as much weight as the female participants during the walking. Click on the figure for a larger version.

The amount of plasma in the men's blood decreased, in the women it increased.

Part of the explanation is that the men drank less during the walking than the women. Both men and women drank on average just over 2 litres, but the table shows that if you compare the amount of fluid drunk with bodyweight, then men drank significantly less than women.

At the end of the four-day march 34 percent of the men had dehydration symptoms: their bodyweight had decreased by more than 2 percent. Among the women only 12 percent lost so much weight.

In addition, 27 percent of the men had hypernatremia by the time they reached the finish line. Hypernatremia is an indication of dehydration.

So men are four times more likely to suffer from dehydration as women. This is partly because men drink less, but also partly because men lose more fluids through sweating. The researchers estimate that women lose about 2.9 ml water per kg bodyweight per hour, and men 3.7 ml.

"Our findings suggest that sex must be considered when providing fluid replacement advices and/or guidelines", the researchers conclude.